Related Field
The present invention relates to a thin film solar cell. In particular, the present invention relates to a thin film solar cell that has one or more diffractive optical supercell structures with the purpose of enhancing light trapping.
Description of Related Art
Solar panels are made of a series of solar cells. These are used to capture sunlight and convert it into electrical power. With the on-going move towards renewable energy, solar panels are becoming of increasing interest to consumers and businesses alike. However, the cost of solar panels is still relatively high and the efficiency relatively low.
To make solar panels more accessible, there has been a drive to reduce costs and improve efficiency. To this end, thin film solar cells are increasingly being used. These devices have an active layer that has a reduced thickness. This means that processing and material costs are reduced, while the overall device efficiency can be maintained. To maintain the efficiency of thin film devices, light trapping structures are used to effectively couple light into guided modes of the thin film, thus facilitating efficient absorption.
Recent work has shown that the use of diffractive optical structures provides a promising light trapping technique for thin-film solar cells. Diffractive structures or gratings can be used to excite quasi-guided modes in the absorbing film, providing strong absorption enhancement at the resonant wavelength. As an example, a simple grating has been used in a solar cell to act as a surface coupler. The grating has a period corresponding to the wavelength of light, i.e. 500-1000 nm for silicon-based solar cell applications. Such a grating relies on few diffraction orders, mainly the first, to couple light into the thin film. This gives a very narrow-band and strongly angle-dependent enhancement, so only a limited scope for light trapping.
To increase the bandwidth and the number of angles that can be coupled into guided modes, a larger period grating can be used. In this case, higher orders of the grating are used to excite multiple modes in the thin film. Simple gratings diffract most of the energy into the first diffracted order. However, this cannot excite a guided mode if the period is larger than the wavelength. Hence, most of the incoming power is lost and not successfully diffracted into a guided mode.